Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion.

The reduction of the boryl-substituted Sn II bromide {(HCDippN) 2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K 2 [Sn 4 {B(NDippCH) 2 } 4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K + cations or oxidation leads to contraction or cleavage of the Sn 4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn 4 {B(NDippCH) 2 } 4 H 2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn 4 {B(NDippCH) 2 } 4 ] 2- dianion.